The use of water as a solvent for lacquer applications has increased greatly in recent years. A deciding factor in the development of this technique is the role played by environmental considerations. For instance, this technique enables the use of organic solvents for the application of the lacquer to the substrate to be reduced greatly. A marked reduction in the emission of volatile constituents (so-called VOCs, volatile organic compounds) which favour ozone destruction and improved working conditions for users are associated with this. It is furthermore possible largely to dispense with the combustion of exhaust air from lacquering installations, resulting in cost savings.
For conventional lacquer systems, that is to say lacquer systems with which organic solvents are used for the application of lacquers to the substrate which is to be lacquered, a range of catalysts have been described which accelerate the reaction of (poly)ols with (poly)isocyanates to form polyurethanes. Depending on the desired working life, a catalyst having a reactivity which is suitable in each case can be selected in order to adjust the desired lacquer properties in accelerated manner. Catalysts based on tin compounds, in particular based on tin(IV) compounds, are among the catalysts which are typically used. Dibutyltin dilaurate, DBTL, is particularly preferred here. This compound is probably the most frequently used catalyst for so-called two-component polyurethane (2-K PU) lacquer applications. Tin salts or organotin compounds lead to the rapid reaction of isocyanates with alcohols or polyols. Alternatively, bismuth compounds and zinc compounds may also be utilised. These normally have a longer pot life and reaction time than the tin compounds. The use of zirconium chelates such as zirconium(IV) acetylacetonate has also been described. These have been described in Journal of Coatings Technology 2002, 74(930), 31–36, inter alia. Florio in Paint and Coatings Industry 2002, 16, 80, for example, gives an up-to-date overview of common catalysts. However, other typical polyurethane catalysts such as, for example, iron(III) acetylacetonate or corresponding compounds of nickel or of cobalt are not considered for use in light-fast lacquers because these catalysts generally form coloured complexes.
By contrast with conventional, solvent-containing systems, when water is used as a solvent for lacquer applications additional considerations should also be taken into account. An overview of these problems is provided in W. Blank Progress in Organic Coatings 1999, 35, 19, for example, as well as in WO 98/41322 and the literature cited therein.
In these lacquer systems the relative reaction rate of the isocyanate with alcohols must be considered by comparison with the reaction rate with water. The reaction of isocyanates with water leads to the formation of carbamic acid derivatives which subsequently react off to form the underlying amine and carbon dioxide. The carbon dioxide which is formed can manifest itself in the film as blistering, adversely affecting the film quality. For this reason carbon dioxide formation is undesirable. An amine which can react off with free isocyanate to form ureas is released from the decarboxylated carbamic acids. Excessive urea formation in turn manifests itself in a reduction in the pot life of the system and typically in loss of surface gloss and deterioration in the lacquer properties following the lacquering operation.
The reaction of isocyanates with water is consequently undesirable on account of the secondary reactions and the rapid loss of properties. By contrast with the non-catalysed lacquering system, therefore, in order to avoid a deterioration in properties, the reaction of water with isocyanates should not be preferential. It is desirable that the reaction of (poly)ol with the isocyanate component is preferential.
Furthermore, prior art catalysts in general have only a finite life in water-based systems, that is to say the catalyst is hydrolysed with greater or lesser rapidity by the action of water. This is true to a particular extent in the case of the tin(IV) compounds such as DBTL, already mentioned, which are popularly used in conventional systems, or also in the case of bismuth carboxylates such as, for example, bismuth(III)-2-ethylhexanoate (K-Kat, King Industries, Norwalk, Conn., USA), as also described in WO 00/47642.
Moreover, most industrially used polyol components for water-based 2-K PU applications have available carboxyl groups (neutralised with tertiary amines) which serve to hydrophilise the binder, that is to say serve to enable the polyol component to be incorporated in water. As a result of complexing, these carboxyl groups may under certain circumstances cause an inhibition of the catalytic activity of organotin compounds utilised as catalysts for 2-K water-based systems. This applies to all highly charged Lewis acids such as, for example, titanium(IV) compounds, zirconium(IV) compounds, and like compounds. A catalyst which is to be universally usable with a multiplicity of hydrophilised polyisocyanates and hydrophilised binders cannot be permitted to exhibit these interactions with the hydrophilising agents.
Tin and zirconium compounds have been described recently as catalysts for 2-K water-based systems. According to WO 98/41322 zirconium(IV) acetylacetonate should contribute towards accelerated curing of the 2-component polyurethane lacquer film in water-based systems, without lacquer films obtained in this case being of lower quality as regards gloss and haze than in the non-catalysed case. However, WO 98/41322 gives only examples of lacquer systems which are based on conventional organic solvents. No examples of lacquers obtained by the reaction of hydrophilised binders (polyols) with hydrophilised polyisocyanates (therefore those in which account must be taken of an interaction between the hydrophilising agent and the catalyst) are indicated. The teaching of WO 98/41322 furthermore describes the addition of a complexing agent (acetylacetone) which, in order to set the catalysis in motion, must not volatilise until the lacquer film has been applied. This procedure is necessary in order to keep the activity of the catalyst to a minimum during the pot life. Without the use of this complexing agent the pot life would fall to an unacceptable, impracticable level. The complexing agent has the disadvantage that it constitutes an additional volatile organic component, leading to renewed environmental pollution and adversely affecting working conditions for the user.
The object of finding a catalyst for the acceleration of the reaction of isocyanates with alcohols or polyols in the presence of water or generally for accelerating the curing of water-based 2-K PU systems has therefore arisen. The general lacquer properties dependent on the working time should not suffer as a result of the utilisation of the catalyst, and the pot life should not be shortened. Ideally, the pot life is not influenced by the presence of the catalyst. The catalyst should be hydrolytically stable and should have sufficient activity while active substance quantities are yet very low. Account should also be taken of ecological and economic considerations (price).